Exciting new research has promoted a vital sub-field of speech perception research concerned with describing the function of categories in the development and maintenance of language- appropriate perception. Recent work has suggested that at least part of the formation and function of phonetic categories is a result of general perceptual categorization mechanisms not specific to speech or language. Thus, there now appears to be opportunity for an integration of general categorization research with work on first and second language acquisition. Unfortunately, much of what is known about perceptual categorization has been derived from examination of categories that are fundamentally different from phonetic categories. Moreover, it is empirically difficult to examine influences of categorization using speech stimuli because it is extraordinarily difficult to determine a detailed history of experience. Pilot work by the PIs has suggested the utility of using complex non- speech sounds in probing the learning mechanisms that drive auditory categorization. These sounds can be synthesized to mimic complexities of phonetic categories and distributions of stimulus presentation can be theorectically derived to model aspects of phonetic categories while maintaining full experimental control over experience. The main goals of this work are threefold. The first goal is to provide a detailed database of the formation and structure of complex auditory categories. There is a dearth of research in this area and the proposed work will be useful in developing a taxonomy of auditory learning and testing extant models of general perceptual categorization (which have been based primarily on data from visual tasks). Experiments using explicit and incidental learning procedures will map the development of categorical response structures as listeners gain experience with novel stimuli. The second goal is to compare the resulting structures that arise from these categorization tasks to structures typical of speech categories such as categorical perception and the "perceptual magnet" effect. The third goal is to develop efficient methods of exposure and training to teach non-native contrasts to second-language learners. Learning the sound contrasts for a non-native language is an extremely difficult task. Exposing the mechanisms of complex category learning could illuminate potential aids to training individuals to discriminate these complex speech categories. These aids could extend easily to other complex learning tasks such as musical training, acoustic warning systems or auditory data displays.